This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Airway smooth muscle has been implicated in the excessive bronchoconstriction of asthma. The present study addressed the question of whether the episodic nature of urban exposure to oxidant air pollutants contributes to the rise in childhood respiratory diseases by altering postnatal lung development.